Which of these is a digital broadcast system? A.
NTSC
B.
PAL
C.
DTMB
D.
SECAM

All but one analog television system began as black-and-white systems. Each country, faced with local political, technical, and economic issues, adopted a color television system which was grafted onto an existing monochrome system, using gaps in the video spectrum (explained below) to allow color transmission information to fit in the existing channels allotted. The grafting of the color transmission standards onto existing monochrome systems permitted existing monochrome television receivers predating the changeover to color television to continue to be operated as monochrome television. Because of this compatibility requirement, color standards added a second signal to the basic monochrome signal, which carries the color information. The color information is called chrominance with the symbol C, while the black and white information is called the luminance with the symbol Y. Monochrome television receivers only display the luminance, while color receivers process both signals. Though in theory any monochrome system could be adopted to a color system, in practice some of the original monochrome systems proved impractical to adapt to color and were abandoned when the switch to color broadcasting was made. All countries used one of three color systems: NTSC, PAL, or SECAM.

Explanation:

answer; always; never;